Biological creatures with unique surface wettability have long served as a source of inspiration for scientists and engineers. More specifically, certain beetle species in the Namib Desert have evolved to collect water from fog on their backs by way of wettability patterns, which attracted an ongoing interest in biomimetic studies. Bioinspired materials exhibiting extreme wetting properties, such as superhydrophilic and superhydrophobic surfaces, have attracted considerable attention because of their potential use in various applications. Combining these two extreme states of superhydrophilicity and superhydrophobicity on the same surface in precise two-dimensional micropatterns opens exciting new functionalities and possibilities for a wide variety of applications. In this review we briefly describe the water-harvesting mechanisms of a genus of Namib Desert beetle, Stenocarpa, consisting of the theory of wetting and transporting. Then we describe the methods for fabricating superhydrophilic-superhydrophobic patterns and highlight some of the newer and emerging applications of these patterned substrates that are currently being explored. Finally, we provide conclusions and outlook concerning the future development of bioinspired surfaces of patterned wettability.
All living creatures in the world, including human beings, require water for their survival. However, over the past few decades, water shortage has been one of the most serious global crises, which greatly threatens the survival of billions of people in some arid and developing countries (Gao & You 2015; Hybel et al. 2015; Chaturvedi et al. 2016; Mekonnen & Hoekstra 2016; Zhu et al. 2016). Actually, water is the most abundant resource in the natural environment, but salt water is almost 96.54% of all the water on the Earth. The freshwater available directly to people is only 0.36%, mainly arising from frozen glaciers, polar ice caps and unfrozen groundwater, of which a small fraction is present on the surface or in the air (Kalogirou 1997; Gohari et al. 2012; Gorjian et al. 2014; Gorjian & Ghobadian 2015). The water problems arising from fast-growing economic activity and poor management of water resources have attracted widespread attention worldwide as they affect human health and slow down economic development (Jurado et al. 2012; Vengosh et al. 2014; Gorjian & Ghobadian 2015; Hanna-Attisha et al. 2016; Ge et al. 2017). Therefore, obtaining clean water is essential for human society as well as maintaining the diversity of our living environment.
How to obtain more water is an issue that needs to be urgently addressed (Ridoutt & Pfister 2010; Ahmad & Deog-hwan 2015; Carroll et al. 2015; Gilbertson et al. 2015). Nowadays, seawater desalination technology has been extensively applied to solve human water shortage problems, to reduce dependence on precipitation cycles and increase the availability of traditionally unusable water. However, although desalination is a reliable technology for creating freshwater, dramatically high costs and reliance on energy are needed for this method, which have limited further application (Fritzmann et al. 2007; Khawaji et al. 2007; Lee et al. 2011; Menachem & Phillip 2011; Zhao et al. 2012; Grubert et al. 2014; Lutchmiah et al. 2014).
Recently, much attention has been paid to metal-organic frameworks (MOFs) – materials which adsorb water from the air (Alina et al. 2019; Suh et al. 2019; Hanikel et al. 2020; Logan et al. 2020; Pan et al. 2020). MOFs are a new type of porous material, connected by metal nodes and organic ligands, which have unprecedentedly high surface areas and porosities (Li et al. 2019). Although they have great potential to collect water from the air, the application of these materials in the field of water harvesting presents some challenges as most MOFs have poor abilities to withstand structural degradation in the presence of water or vapor (Schoenecker et al. 2012). However, with the deepening of the research, MOFs materials will definitely play an important role in the field of water collection in the near future.
How to collect water from the air in dry areas
Increasing biomimetic research shows that the solution to the global problem of water availability could be found in nature (Schemenauer & Cereceda 1991; Schemenauer & Cereceda 1994; White et al. 2013; Chen et al. 2018; Zhou et al. 2018). Indeed, over 4.5 billion years of evolution and natural selection, natural creatures have evolved to take advantage of minimum resources to achieve maximum function (Woodyer et al. 2004; Peisajovich 2012; Gou et al. 2014). In regions where water has been a limited resource for centuries, nature has adapted to provide survival methods for species inhabiting the ecosystems concerned. For example, certain beetle species in the Namib Desert have evolved to collect dew and fog from the air on their backs by way of wettability patterns (Parker & Lawrence 2001). This has been described from the study of the elytra of beetles from the genus Stenocara and has attracted increasing interest in biomimetic studies (Ju & Zheng 2014; Chen et al. 2018; Comanns 2018; Li & Guo 2018). Fog brings water in the form of minute droplets and fog events occur on approximately 30 days per year in the Namib Desert, which therefore represent a predictable source of water for the Namib Desert beetles (Seely 1979; Lancaster et al. 1984; Pietruszka & Seely 1985). In a foggy dawn, the Stenocara beetle tilts its body forward into the wind to capture small water droplets in the atmosphere. After these small droplets coalesce into bigger water droplets, they roll down into the beetle's mouth, providing it with a fresh morning drink. The structures behind this process are believed to be an array of surface bumps topped with hydrophilic spots (about 100 μm in diameter) on a superhydrophobic background; small water droplets carried by the morning fog settle on the hydrophilic peaks of the smooth bumps on the wings of the beetle and form fast-growing droplets; once the weight of a growing droplet is sufficient to overcome the binding forces of the hydrophilic region, it rolls down toward the beetle's head.
With the inspiration of the fog-collection principle of the patterned hydrophilic-hydrophobic surface of desert beetles (Hamilton & Seely 1976; Garrod et al. 2007; Cao et al. 2015; Park et al. 2016), biomimetic materials with smart structures are widely fabricated for water collection (Zhai et al. 2006; Lee et al. 2007; Rizzello et al. 2009; Kang et al. 2010; Piret et al. 2010; Zhu et al. 2010; Li et al. 2012; Zhang et al. 2015). In this review, the recent research on the fog-collecting materials inspired by Namib Desert beetles, particularly on the mechanisms of water capture, the fabrications, functions, applications, and new developments in recent years are summarized. The theoretical basis is first introduced to help understand the wetting behaviors in water collection, and the Young's equation, Wenzel model, Cassie model are described. This is followed by a discussion on the water-collecting mechanism of Namib Desert beetles and their bioinspired materials. Finally, the remaining challenges and promising breakthroughs concerning the future development of bioinspired materials are presented.
BASIC THEORIES AND WATER-COLLECTING MECHANISMS
Here, r (r > 1) means the ratio of the real contact line to the projected contact line of the portion of solid in air. It is shown in the equation that the combined effect of surface morphology (r) and the surface chemical composition () are observed to be decisive factors to influence the apparent contact angle. Nevertheless, the absolute value of might be larger than 1 in some surfaces with high roughness or porous structures. So as the Wenzel model is insufficient, the Cassie model will be used to introduce such wetting behavior instead.
The collection of water by bioinspired micropatterns is becoming increasingly important and has attracted increasing attention. To explain the water collection mechanism, researchers have proposed many models in recent years, such as driving forces, resistance forces and hanging ability. These models can help people to have a deep understanding of the process of the collection of water by bioinspired microfibers and direct the design of bioinspired microfibers for better water collection.
As one of the world's oldest and driest deserts, the Namib Desert formed 80 million years ago and is now located in the Namib-Naukluft Park in Africa (Geyh & Heine 2014; Hamdan et al. 2015). The annual rainfall is less than 13 mm (0.5 inch), although at night, fog coming from the sea brings some water, which is vital for the native flora and fauna to survive in (Jankowitz et al. 2008; Murray et al. 2016). After thousands of years' evolution, a host of animals are well adapted to the arid conditions.
It is worth noting that fog collection and vapor condensation might not be the same process. Fog harvesting depends on the collision between the tiny fog droplet and the fog-collecting surface, which is irrelevant to phase transition (Cao & Jiang 2016). Dew is the condensation of water vapor into liquid droplets on a substrate; it is a common natural phenomenon and considered to be the inverse of evaporation (Beysens 2006).
In order to determine whether dew water collection is also possible for desert beetles, Guadarrama-Cetina and Mongruel (Guadarrama-Cetina et al. 2014) investigated the infra-red emissivity, and the wetting and structural properties, of the surface of the elytra of a preserved specimen of the beetle. They performed scanning electron microscopy on histological sections and determined the infra-red emissivity using a scanning pyrometer; the emissivity measured was close to the black body value. The results showed that the characteristics of these beetle elytra prompt dewdrop formation.
FABRICATION OF BIOINSPIRED ARTIFICIAL NAMIB DESERT BEETLE-LIKE SURFACE
Beetles in the Namib Desert have unique morphologies and surface microstructures for highly efficient water collection. Motivated by the Stenocara beetle, many hydrophilic-hydrophobic patterned surfaces have been constructed. Generally, on such patterned wettable surfaces, there are at least two portions with different wettabilities ((super)hydrophilicity and (super)hydrophobicity). Next, we briefly introduce the main existing techniques for the fabrication of bioinspired hydrophilic-hydrophobic patterned surfaces.
Zahner et al. took advantage of the selective illumination of UV light via a photomask to fabricate superhydrophobic-superhydrophilic patterned porous polymer films (Zahner et al. 2011). The method they adopted was based on the preparation of a hydrophobic thin porous polymer film, which was then modified by UV-initiated surface photografting through a photomask (Ranby 1992; Rohr et al. 2003). In order to create a superhydrophilic micropattern with defined geometry on the superhydrophobic polymer background, they first prepared superhydrophobic, microporous poly(butyl methacrylate-co-ethylene dimethacrylate) films by UV-initiated radical polymerization of a prepolymer mixture containing monomers, porogens, crosslinkers, and a UV initiator. Then the film was wetted with a photografting mixture composed of a methacrylate monomer, benzophenone as the initiator, and a mixture of tert-butanol and water. Finally, the polymer surface was irradiated with UV light through a photomask.
Such patterns with extreme differences in the wettability between superhydrophilic and superhydrophobic areas can be applied to microfluidic channels, water collection, cell growth and site-selective immobilization of functional materials (Wu & Zhang 2015; Tserepi et al. 2016; Yu et al. 2017; Kostal et al. 2018). To further develop such functional interfaces, it is critical to enhance contrast in the wettability on the patterned surfaces. Nishimoto et al. prepared a superhydrophobic surface with an extremely high static water contact angle by surface modiﬁcation with self-assembled monolayers (SAMs) of octadecylphosphonic acid on rough nanostructured anatase TiO2 surfaces (Nishimoto et al. 2014). The superhydrophobic TiO2 ﬁlms could be conveniently patterned with UV light to produce superhydrophobic-superhydrophilic patterns with an extremely high wettability contrast (∼170°), which could be employed to selectively ﬁll superhydrophilic areas with water-based functional materials.
Additionally, Bai et al. designed a novel kind of surface with star-shaped wettability patterns to improve the water-collection efficiency with control over the surface wettability. By integrating Laplace pressure gradient and surface energy gradient, the patterned surface can quickly drive tiny water droplets toward more wettable regions to avoid them being lost in the wind. The results showed that this type of surface is more efficient in water collection than uniform superhydrophilic, uniform superhydrophobic, or even circle-patterned surfaces.
Surfaces with star-shaped wettability patterns can be fabricated following the procedures in Figure 3. First, they fabricated a superhydrophilic surface by depositing TiO2 slurry onto a glass substrate via a spin-coating method, then the film was treated with fluorinated alkyl silane (FAS) to change the wettability from superhydrophilic to superhydrophobic. Subsequently, the circular-shapes pattern and 4-, 5-, 6-, 8-pointed star-shapes of photomask were utilized to construct the superhydrophilic pattern on superhydrophobic surface via selected exposure to UV light. Water-collection experiment results showed that the shape and size of the pattern is crucial for enhancing the water-collection efﬁciency on patterned surfaces. These investigations may provide insights in designing and developing materials with controllable wettability for highly efﬁcient water- or liquid-collecting technology (Bai et al. 2014).
Moazzam et al. utilized the negative photolithography and biopolymer of polydopamine (PDA) method to produce a porous membrane surface with contrasting wettabilities by creating hydrophilic patterns (nanoscale PDA-coated SU-8 bumps) on the hydrophobic background of polypropylene (PP) membranes (Moazzam et al. 2018). The fabrication process involves four main steps as shown in Figure 4. First, the PP microﬁltration membrane (PPMM) was bound to a silicon wafer using positive photoresist as an intermediate adhesion layer. This step was followed by spin coating an approximately 200 um thick SU-8 layer on the PPMM surface. In order to make the SU-8 surface superhydrophilic instead of hydrophobic surface with the contact angle of 90°, they used a PDA coating which reduces the free energy of the surface and creates an SU-8 surface with contact angle of 10°. The next step was to transfer the mask geometry to the SU-8 photoresist surface using a negative lithography process. After that, the sample was immersed in acetone to separate the PPMM from the silicon wafer and obtain the product. They then investigated the fog-harvesting performance of different surfaces and found that the patterned coated SU-8 surface exhibited excellent performance.
In actual applications, the patterned area using the conventional fabrication method is not suitable for mass fabrication. So, it is desirable to find a method for continuous fabrication of the surface that mimicks the Stenocara beetle's back. Lee et al. proposed a continuous fabrication method to make a bioinspired patterned surface by using roll-type photolithography for potential applications to real-time air-monitoring systems (Lee et al. 2014). The fabrication procedure consists of four detailed processes as follows: molding, deep UV etching, surface treatment, and roll-type photolithography for selective exposure. Low contact-angle hysteresis is not suitable for a water-collecting surface because the water droplets roll away before they get bigger by aggregating and the wettability of solid surface can be controlled by surface topography and/or surface chemistry (Choi et al. 2008). So they tested various materials and micro/nanogeometries to obtain optimized water-contact characteristics for a patterned surface. Water-collection experiments showed that the hydrophilic surface array with a width of 3 mm and a spacing of 5 mm on the hydrophobic surface demonstrated a maximum water-collection performance when compared with other cases. A significant feature of this method is that it allows rapid, permanent changes in surface wettability from superhydrophobic to superhydrophilic to define local wettability without complex time-consuming processing.
However, due to the photocatalytic degradation of low-energy hydrocarbon groups or the corresponding micro/nanostructure damage and other possible circumstances, the practical applications of patterned surfaces with hybrid wettability are likely to be affected for high-efficiency water harvesting. What is more, the bioinspired materials usually function well in the laboratory, completely ignoring the multiple factors in the external environment, let alone the continuous mass production; therefore, how to produce hydrophilic and hydrophobic patterned surfaces in a simple and efficient production method is what we should consider in the future.
Composite technology of materials with different wettabilities
Generally, on such a patterned wettable surface, there are at least two portions with different wettabilities. One possibility for making a composite surface with pattern dimensions is simply pressing together materials with different wettabilities. Based on this method, Cao et al. successfully constructed a Janus system by compositing hydrophilic cotton absorbent and hydrophobic copper mesh, demonstrating an enhanced efficiency for water harvesting (Cao et al. 2015). They first selected the hydrophobic mesh with a smaller pore diameter (<50 um) and the hydrophilic cotton absorbent was selected due to its high potential water capacity, cost efficiency and loose structure. Then the processed Janus system also possessed an intrinsic surface pattern, i.e., the hydrophilic cotton in the pores surrounded by the hydrophobic copper wires. Their experiments with collecting water showed that the hydrophilic cotton absorbent only collected 0.14 ± 0.02 g water in 5 min. The Janus system can gather up to as much as 0.310.03 g water, 1.3 times higher than that of a hydrophobic mesh, showing an excellent water-collecting ability. By further improvement, they designed and fabricated a cylindrical Janus fog collector, changing the 2D hydrophobic-hydrophilic cooperative system into the 3D ones. The device works efficiently no matter which direction the fog comes from.
Similarly, Yin et al. prepared a hybrid wettable surface by incorporating superhydrophobic copper mesh and pristine hydrophilic copper sheet as shown in Figure 5. Firstly, the polytetrafluoroethylene (PTFE) sheet was adhered under the copper mesh and then the resulting sample was fixed on a translation stage in air. Subsequently, the resulting sample was treated with a femtosecond laser and the ejected PTFE nanoparticles in sizes from tens to hundreds of nanometres adhered to the copper mesh. Finally, pristine hydrophilic copper sheet was tightly adhered. It was found that the surface exhibited fog-harvesting properties with good efficiency and the water-collection rate of the as-prepared surface could be optimized by controlling the mesh number and inclination angle. In addition, the as-prepared samples showed anticorrosion properties during corrosion testing (Yin et al. 2017).
Meanwhile, Gao et al. firstly introduced a hydrophilic-superhydrophobic patterned weft-backed woven fabric (Liu et al. 2016; Gao et al. 2018a, 2018b) fabricated by a facile weaving method with simple textile equipment. Hydrophilic viscose and hydrophobic PP yarns were used to produce the hybrid wettable surface with common commercial agents, which sharply relieved the cost, making it possible to produce the water-collecting materials at a large scale in future (Figure 6). Pre-cleaned viscose yarns and PP yarns were finished by dipping in a hydrophilic agent (60 g/L) and a hydrophobic agent (60 g/L) for 30 min each and baking at 120 °C for 10 min. After being dried in an oven, the hydrophilic viscose yarns were used as the weft yarns and the hydrophobic PP yarns were used as the warp yarns. They wove hydrophilic-superhydrophobic patterned surfaces in proportions of 1:1, 1:3, 1:5, 1:7 (viscose yarns:PP yarns) while the other side was completely superhydrophobic with the semi-automatic loom. When the surface area of all the samples was the same, it turned out that the as-prepared sample featured the best water-harvesting rate of 1,267.5 mg h−1 cm−2 at a proportion of 1:1. More importantly, the fabric could be recycled for 10 times: the weft backed woven fabric remained intact even after 2,000 abrasion tests, and water contact angles exceeded 140° on hydrophobic regions and remained at 0° on hydrophilic regions respectively (Gao et al. 2018a, 2018b).
Additionally, dewetting of thin polymer films has been extensively investigated as a way to generate patterned surfaces. In the last decade, the Neto group has used dewetting of polymer films extensively to generate patterned surfaces for a range of applications, including atmospheric water capture and cell patterning (Thickett et al. 2011; Telford et al. 2012; Manuel et al. 2014; Al-khayat et al. 2017; Telford et al. 2017). As sol-gel films are much more robust than polymer films and can withstand high working temperature and UV irradiation (Lee & Crayston 1993; Brusatin et al. 2000; Figueira et al. 2015), Colusso et al. investigated for the first time the dewetting of bilayers of sol-gel films: the top film is hydrophilic (silica) and the bottom one hydrophobic (CH3-silica), with the objective of producing silicate patterned surfaces (Colusso et al. 2019). A hydrophobic sol-gel silica film was first spin coated onto a silicon wafer and then the surface wettability of the sol-gel film was made hydrophobic by adding hydrophobic methyl groups: a CH3-modified hydrophobic silica solution was obtained by hydrolysis and condensation of a mixture of tetraethoxysilane and methyltriethoxisilane with a molar ratio of 1. Subsequently, a freshly made xerogel of silica solution (1 day aged) was spin coated onto the CH3-silica film and then was exposed to ethanol vapor at room temperature. As the xerogel was fresh, the ethanol vapor penetrated the film and reduced the rate of condensation through a series of reactions. Finally, hydrophobic non-wettable substrate dewetted.
Inkjet printing technology
Inject printing technology is an extremely simple, effective and economical way of surface wettability patterning to produce high resolution patterns with complicated shapes (Shimoni et al. 2014; Sun et al. 2015; Lee et al. 2016). Zhang's group adopted the inkjet printing method to construct the superhydrophilic micropatterns on superhydrophobic surfaces. First, they applied a mussel-inspired ink (consisting of an optimized solution of dopamine) directly by inkjet printing to superhydrophobic surfaces. Afterward, microdroplets of the dopamine solution with micropatterns were obtained on the surface and then superhydrophilic micropatterns with well-controlled dimensions were achieved by the formation of PDA via in situ polymerization (Zhang et al. 2015). Similarly, Zhu et al. achieved the hydrophilic-superhydrophobic patterned surfaces by inspiration from mussels and Namib Desert beetles, as shown in Figure 7. They adopted Cassie-state superhydrophobic substrates to synthesize the composite materials, which can quickly transport water away from the surfaces and then improve water-collection efficiencies. The Cassie superhydrophobic copper foils were firstly prewetted by dichloromethane (DCM), immediately followed by dropping the mussels-inspired hydrophilic and bio-adhesive dopamine solution on the treated surface. DCM will ‘cloak’ dopamine due to their different surface tensions. Along with DCM volatility and self-polymerization of dopamine, hydrophilic PDA patterns were constructed on the Cassie superhydrophobic Cu foils, thus obtaining patterned materials. The sample with about 8.0% hydrophilic area exhibited the most efficient fog-harvesting properties, with a water-collection rate of 5.5 mg min−1 cm−2. This method can be widely applied on amounts of superhydrophobic materials in the Cassie state, such as Fe plate, Al plate, cotton fabric, Cu mesh, and Ni foam, etc. The study greatly broadens the preparation of fog-harvesting materials and its practical application in the field of water collection (Zhu et al. 2018).
Li et al. constructed superhydrophilic micropatterns on a superhydrophobic substrate based on printing an ethanol solution containing a phospholipid onto a superhydrophobic porous polymer surface. The method is compatible with different printing techniques, such as microcontact or inkjet printing. First, the lipid solution was printed in an array pattern onto a thin superhydrophobic porous poly(butylmethacrylate-co-ethylene dimethacrylate) (BMA-EDMA) surface using metal spotting pins. After the lipid array was printed, the polymer substrate was wetted with an aqueous solution, making the created lipid layers on the polymer surface switch from superhydrophobicity to superhydrophilicity. Finally, the surface was dried gently with nitrogen, and the superhydrophilic-superhydrophobic micropatterned surface was obtained. Since lipid layers can also incorporate different bioactive molecules, transmembrane proteins, or other functional lipids, this facile procedure for creating superhydrophilic patterns combined with contemporary printing technology will lead to numerous applications (Li et al. 2012).
Sun et al. also presented a flexible, convenient and low-cost fabrication method for creating superhydrophilic-superhydrophobic patterned surfaces by inkjet printing a sacrificial layer on a superhydrophilic surface. First, a thin aluminum film with hierarchical porous nanostructures was deposited on to a silicon substrate via vacuum deposition. Second, high resolution patterns were inkjet printed on the superhydrophilic surface. The printing ink was a PAA (polyacrylic acid) solution with 30 wt%. After modifying the surface with (FAS) and removing the printed water-soluble deposit, the superhydrophilic-superhydrophobic patterned surfaces were obtained (Sun et al. 2016). Additionally, Jiang's group utilized inkjet printing to fabricate a series of superhydrophobic-superhydrophilic patterned surfaces, which would contribute to the research on droplet transport and water collection in the future (Jiang et al. 2016).
In production, it is difficult to spin-coat a layer of photoresist on a superhydrophobic surface and hierarchical structures are found to be more efficient for water collection than simple patterns (Ju et al. 2012). So, it is imperative to develop a method that can generate high-resolution and large area hierarchical patterns with controllable surface wettability. Using laser ablation to mimic hierarchical surface morphologies has several advantages. First of all, it is a maskless process, which enables direct writing of arbitrary geometries. Moreover, double-hierarchical surface structures can be easy to fabricate (Jörn et al. 2012). Based on this approach, Kostal et al. presented a novel three-step fabrication method to mimic the Namib Desert beetle's elytra. In the first step, a double-hierarchical surface structure was generated on Pyrex wafers using femtosecond laser micromachining, which made it superhydrophilic (water contact angle <10°). In the second step, a Teﬂon-like coating was applied to switch the wetting state from superhydrophilic to superhydrophobic (water contact angle >150°). In the last step, selective ablation was used to locally recover the superhydrophilic state. As experiments in an artificial nebulizer setup have shown, such micropatterns enhance the fog-collection efficiency by nearly 60% compared to blank glass. The method they presented enables the functionalization of a broad range of materials, such as glass. This opens up possibilities for fog-collection surface micropatterns, especially in the field of microfluidic and biomedical devices (Kostal et al. 2018).
Wang et al. also constructed hierarchical patterns with modified wettability and desired geometry on a superhydrophobic film via laser direct writing. As shown in Figure 8, a porous superhydrophobic TiO2 surface was fabricated by a hydrothermal method. After that the TiO2 surface was patterned by laser writing at a high resolution of 300 nm. The laser beam removes the surface structures, making the film smoother and relatively wettable. The patterned ﬁlm can precisely and directionally drive tiny water droplets and dramatically improve the efficiency of water collection with a factor of ∼36 compared with the original superhydrophobic ﬁlm. Such a patterned ﬁlm might be an ideal platform for water collection from humid air and for planar microﬂuidics without tunnels (Wang et al. 2017a, 2017b).
Recently, Lu et al. reported a novel facile physicochemical hybrid method that combines femtosecond laser structuring with hydrothermal treatment to create a patterned wettability surface with a well-arranged hierarchical nanoneedle structure. They first created uniform microstructure square arrays on Ti sheets by a femtosecond laser system. Then, Ti sheets with microstructures were hydrothermally treated in an NaOH solution at 220 °C for 24 hours using an electric oven. Next, the surface component was transformed to TiO2 by immersing the hydrothermally treated titanium sheets in 1 mol/L HCl solution for 10 min, washing with distilled water, and annealing at 450 °C for 1 hour in air using a muffle furnace. For the creation of surfaces with special wettability, polydimethylsiloxane (PDMS) liquid was evaporated onto the TiO2 surface in a preheated muffle furnace. Superhydrophobic surfaces or patterned wettability surfaces with hierarchical micro/nanostructures were fabricated by to the modiﬁcation of PDMS. During the water-collection process under a vapor ﬂow environment, the highest performance was achieved for patterned wettability hierarchical micro/nanostructures, which is 2.2 times that of the untreated Ti surface. Moreover, a uniform water condensation under low humidity (28%) was achieved, which has potential applications in harvesting water from arid environments and in high-precision drop control (Lu et al. 2019).
Hydrophilic-hydrophobic patterned surfaces offer a means of controlling the wetting behavior of aqueous media. This is important for a whole host of technological applications including: cell growth, protein manipulation, the spotting of biomolecules, microfluidics (to control the location and movement of liquids), and the formation of anti-dew/frost-free protective exteriors.
Efﬁcient water collection from a humid atmosphere is critical for creatures living in water-limited areas. The Namib Desert Stenocara beetle uses the surface of patterned wettability on its back to collect drinking water from fog-laden wind. The beetle's back consists of alternating hydrophilic bumps and superhydrophobic channels. Inspired by this surface design, Yu et al. fabricated superhydrophilic-superhydrophobic patterned surfaces on the silica poly(dimethylsiloxane) coated superhydrophobic surfaces, to mimic the function of the beetle's back, via a pulsed laser deposition approach with masks (Yu et al. 2017). The wettability patterns exhibit extreme hydrophobic contrast. Water sprayed on superhydrophobic patterns will form spherical droplets. Most of the droplets bounce and roll on the superhydrophobic regions and eventually adhere to the hydrophilic patterns. In order to improve the water-collection efficiency with control over the surface wettability, Bai et al. designed a novel kind of surface with star-shaped wettability patterns (Bai et al. 2014). The patterned surface integrates a surface energy gradient and Laplace pressure gradient. As a result, this type of surface is more efficient in water collection than uniform superhydrophilic, uniform superhydrophobic, or even circle-patterned surfaces. Recently, surfaces with two-tier hierarchical micro/nanostructures are also predicted to promote high-frequency droplet growth and removal due to the existence of their unique structures (Cho et al. 2017; Wang et al. 2017a, 2017b). From the methods described above, we have found that Laplace pressure determined by surface morphology or chemical composition is crucial to water-harvesting efficiency because it can inﬂuence tiny water droplet condensation and transportation.
Cell adhesion and proliferation is an important physiological process, which is strongly affected by surface topology and chemistry. Moreover, it is important to control cell adhesion to surfaces for biological studies and diagnosis. It has been reported that superhydrophobic surfaces can completely inhibit cell adhesion, while superhydrophilic surfaces will enhance cell attachment (Song et al. 2009). Cell interactions with superhydrophilic and superhydrophobic surfaces fabricated by patterning have been extensively investigated (Oliveira et al. 2011; Oliveira et al. 2014a, 2014b). Ishizaki et al. successfully fabricated a micropatterned superhydrophobic-superhydrophilic surface by plasma chemical vapor deposition and vacuum ultraviolet irradiation. Physicochemical properties of the surface affect cell adhesion and cell-cell interactions. In particular, mouse 3T3 fibroblast cells attached to the superhydrophilic regions in a highly selective manner while cell adhesion was suppressed on superhydrophobic surface. Moreover, the amounts of the protein adsorption on the flat hydrophilic surface were much greater than those on the flat hydrophobic surface (Ishizaki et al. 2010).
Recently, Popova et al. demonstrated the applicability of the droplet-microarray (DMA) platform based on superhydrophobic-superhydrophilic patterning for cell-based high-throughput screenings (Popova et al. 2016,, 2017). The unique feature of the DMA platform is the ability to form homogeneous droplet arrays spontaneously without the need for the pipetting of each droplet. Droplet arrays are formed spontaneously due to the effect of discontinuous dewetting. Biocompatibility of the polymer surface allowed us to adopt such arrays for cell culturing and create arrays of droplets containing cells. DMA slides enable miniaturized screenings of live cells in droplets ranging from 3 to 80 nL in densities from 588 to 4,563 spots per standard microscope glass slide, which corresponds to approximately 6,000 and 50,000 spots per area of a standard microtiter plate, respectively. The single-step pipetting-free seeding results in savings in the pipetting steps, pipetting tips, robotics, and time of the experiment.
Cell-biomaterial interactions have been widely investigated on flat 2D surfaces; however, studies in a 3D environment are more valuable because they mimic in vivo cell microenvironments better. The dimensionality of the system can influence many cell functions, including polarity, morphology, motility, and cell-cell interactions (Baker & Chen 2012; Dolatshahipirouz et al. 2014). Oliveira et al. successfully used polystyrene superhydrophobic surfaces patterned with wettable spots as improved and versatile platforms for high-throughput spheroids formation and drug screening in such in vitro-constructed tissues (Oliveira et al. 2014a, 2014b). The wettability contrast of the chips was used to fix cell suspension droplets in the wettable regions and evaluated on-chip drug screening in 3D environment. A fibroblast (L929) and an osteosarcoma cell line (SaOs-2) were used for spheroids formation and drug screening studies. It was previously shown that protein adsorption in the wettable regions of the chips is higher than in the superhydrophobic parts. Moreover, cell adhesion and proliferation were also diminished in the superhydrophobic parts of chips made of different polymers (Ueda & Levkin 2013; Dolatshahipirouz et al. 2014).
Droplets sitting on surfaces with different wetting states will present different contact areas, contact angles and contact angle hysteresis. Hence, it is possible to control droplet motion and liquid transportation by tuning surface wettability. In fact, liquid transportation is an active area for researchers. Brochard reported the motions of droplets on solid surfaces induced by chemical or thermal gradients, in which the Marangoni effect plays an important role (Brochard 1989). Whitesides & Chaudhury ﬁrst reported the uphill movement of a water droplet on a surface of gradient wettability (Whitesides & Chaudhury 1992). The self-motion of the droplet is driven by the imbalanced forces due to the gradient surface tension acting on the solid-liquid contact lines of the droplet. Lorenceau & Quéré reported the self-propelling behavior of wetting silicone oil droplets on a conical ﬁber, in which the driving force is shown to be a gradient of the Laplace pressure of the asymmetric droplets (Lorenceau & Quéré 2004).
Usually, in order to transport liquids across solid surfaces, the construction of a gradient in the interfacial tension is critical at the front and rear ends of the droplet acting at the liquid-solid-vapor interface. It has been demonstrated that surface tension heterogeneity-induced driving force can be used to guide water motion on ﬂat surfaces. Wang et al. reported a novel method to prepare a one-way oil-transport fabric and its application in detecting liquid surface tension (Wang et al. 2015). This functional fabric was prepared by a two-step coating process to apply ﬂowerlike ZnO nanorods, ﬂuorinated decyl polyhedral oligomeric silsesquioxanes, and hydrolyzed FAS on a fabric substrate. Upon one-sided UV irradiation, the coated fabric shows a one-way transport feature that allows oil ﬂuid transport automatically from the unirradiated side to the UV-irradiated surface, but it stops ﬂuid transport in the opposite direction. The fabric still maintains high superhydrophobicity after UV treatment. The one-way ﬂuid transport takes place only for oil ﬂuids with a speciﬁc surface tension value, and the ﬂuid selectivity is dependent on the UV treatment time. Changing the UV irradiation time from 6 to 30 hours broadened the one-way transport for ﬂuids with surface tensions from around 22.3 mN/m to a range of 22.3–56.7 mN/m. For microﬂuidic systems, it is important to precisely control the liquid ﬂow within microchannels.
CONCLUSION AND OUTLOOK
Research involving superhydrophilic and superhydrophobic surfaces actively started only about a decade ago. Since then, many different techniques and materials to produce both types of surfaces have been developed, and most of the research results have already been commercialized or show a great potential application for solving practical problems. Ingenious integration of different special wettabilities provides effective ways of uniting the advantages of the wettabilities, pioneering a new way to develop advanced interface materials. Over the last few years, a number of methods allowing for the fabrication of superhydrophilic-superhydrophobic patterned substrates have been introduced. The key to integrating different super wettabilities is to take advantage of the surface energy difference between two types of surfaces. As shown in this progress report, the trend is now shifting toward exploring better preparation methods and the development of new applications that use the unique properties of such hybrid patterns.
Although the current micropatterned surfaces have shown enhanced efﬁciency in vapor/fog collection, part of the theory and mechanism still remains unclear and should be further investigated. Detailed ﬂuid behaviors, particularly tiny-droplet generation and collection on wettability boundaries should be observed microscopically to determine the functions of wettability difference. Deeper understanding of ﬂuid behaviors at the wettability boundary of hybrid surface should instruct researchers to develop advanced superwettability integration. In addition, the application of research results requires us to consider many practical factors that are often overlooked in the research process, such as technical applicability and scalability, ease of setup, cost effectiveness, stability, and durability. With the continuous progress of science and engineering technology, we believe that superhydrophilic-superhydrophobic micropatterns have a promising future in improving the livability and sustainability of our plant.
This project is supported by the Natural Science Foundation of China (50772131) and National High-tech R&D Program of China (863 Program) (2001AA322100), a grant from the Fundamental Research Funds for the Central Universities (2010YJ05) and the Key projects of the Ministry of Education (106086).